1. Field of the Invention
The present invention relates to a CDMA (Code Division Multiple Access) multi-band mobile terminal, and more particularly, to a GPS (Global Positioning System) receive sensitivity-improving apparatus and method in a CDMA mobile terminal, which improves a GPS receive sensitivity by preventing a reference frequency variation that may be caused by a thermal transient in the CDMA mobile terminal.
2. Background of the Prior Art
FIG. 1 is a block diagram of a GPS receiver inducing a reference frequency variation according to a thermal transient in the conventional mobile terminal.
As shown in FIG. 1, a baseband module 100 processes transmitted/received baseband signals. For example, the baseband module 100 performs channel coding and spreading with respect to transmission data, and performs despreading and channel decoding with respect to a received signal.
A frequency combiner 110 combines desired frequency band signals based on a reference frequency generated by a reference frequency generator 112. A transmitter 120 turns on CDMA/PCS (Personal Communication Service) Power Amplifying Modules (CDMA/PCS PAMs) 122 and 124 for CDMA/PCS transmission in a CDMA/PCS mode. The transmitter 120 performs only CDMA/PCS transmission, not GPS transmission, irrespective of modes.
A receiver 130 performs CDMA/PCS reception in a CDMA/PCS mode, and performs GPS reception in a GPS mode. Generally, the receiver 130 includes a plurality of filters to thereby filter received signals. It should be noted that the CDMA/PCS PAMs are turned off in a GPS mode.
An amplifier 140 amplifies baseband signals processed by the baseband module 100, and then transmits the amplified signals to the CDMA/PCS PAMs of the transmitter 120.
In general, a receive sensitivity is widely used as an indicator for indicating a performance of a GPS receiver. When a mode is converted from a CDMA/PCS mode into a GPS mode, a receive sensitivity of the GPS mode is measured. The GPS receive sensitivity is considerably susceptible to a thermal transient of the reference frequency generator 112.
In general, a receive sensitivity variation caused by a temperature variation is usually proportional to a frequency drift (or variation) caused by the temperature variation. Thus, the frequency drift (FD) of a mobile terminal can be expressed as the following Equation 1.FD(Hz/sec)=S(ppm/° C.)×G(° C./sec)×F(Hz)  Eq. (1)                where, S is the temperature stability of reference frequency generator, G is the thermal gradient within the mobile terminal (° C./sec), and        F is the center frequency (Hz).        
That is, the GPS receive sensitivity is affected by the temperature stability of the reference frequency generator and a PCB (printed circuit board) arrangement, and so on.
When a mode is converted from a CDMA/PCS mode into a GPS mode, the receiver 130 performs GPS reception. At this time, the CDMA/PCS PAMs are turned off. That is, the CDMA/PCS PAMs are turned on in the CDMA/PCS mode but are turned off in the GPS mode. Accordingly, a thermal transient is generated in the reference frequency generator 112 at the mode conversion.
The thermal transient of the reference frequency generator 112 induces a variation in a GPS receive sensitivity. That is, a temperature variation in the CDMA/PCS PAMs affects the performance of the GPS receiver. Moreover, the temperature variation in the CDMA/PCS PAMs further deteriorates a GPS receive sensitivity in a weak electric field areas of the CDMA/PCS communication system.
For solving this problem, the CDMA/PCS PAMs and the reference frequency generator 112 are arranged on the printed circuit board (PCB) in such a way that they are physically spaced apart from each other by as much distance as is reasonably possible. However, such an arrangement of the CDMA/PCS PAMs and the reference frequency generator 112 becomes increasingly difficult as mobile terminals become more compact and decrease in size and weight.
FIG. 2 is a state diagram illustrating a GPS receiving operation state in a conventional mobile terminal.
Generally, in order for a CDMA multi-band mobile terminal to be able to receive a GPS signal while in a CDMA/PCS band, a base station must first command the CDMA multi-band mobile terminal to move to a GPS band.
In response to the command, the mobile terminal converts its frequency band into the GPS band to thereby receive a GPS signal. After receiving the GPS signal, the mobile terminal returns to its original frequency band, that is, a CDMA/PCS band. A state transition sequence in the mobile terminal is illustrated in FIG. 2.
Referring to FIG. 2, the mobile terminal turns on CDMA/PCS PAMs of a transmitter and a CDMA/PCS module of a receiver so as to perform transmission/reception in a CDMA/PCS mode, that is, a CDMA/PCS band.
Accordingly, the transmitter amplifies CDMA/PCS transmission power, and the receiver receives CDMA/PCS signals. At this time, a frequency combiner combines CDMA/PCS frequencies.
If a base station commands conversion to a GPS mode, the mobile terminal converts its frequency band into a GPS band and then receives GPS signals. At this time, the CDMA/PCS PAMs of the transmitter are turned off accordingly as CDMA/PCS frequency emission is terminated.
That is, since transmission is not performed in a GPS mode, the GPS/PCS PAMs are turned off in the GPS mode. In addition, the receiver receives GPS signals, and the frequency combiner combines GPS frequencies.
When the mobile terminal returns to a CDMA/PCS mode, the CDMA/PCS PAMs and the CDMA/PCS module are all turned on.
In the meanwhile, when the mobile terminal is state-transitioned from a CDMM/PCS mode into a GPS mode for GPS reception, the CDMA/PCS PAMs of the transmitter are turned off to thereby induce a temperature variation therein.
The temperature variation causes a thermal transient in the reference frequency generator to thereby deteriorate the GPS receive sensitivity. Such a deterioration of the GPS receive sensitivity is more seriously generated in a weak electric field area requiring a high transmission power or in a high-temperature area.